Water reducers calcium aluminate cements

Other types of admixtures used with calcium aluminate cements include water reducers and superplasticizers (Section 11.4), which also act as retarders, and thickening agents, such as carboxymethylcellulose. Complex formulations may be used for special purposes for example, a ready-mix mortar for high-performance road repair might contain 55% Ciment Fondu and 45% sand, with glass fibre, aluminium powder, Li2C03, sodium gluconate and methyl ethyl cellulose (M93). 

On the other side the distinguishing of acid corrosion has the justification Portland cement paste is not stable in water solution with pH lower than 10, or even 10.5 [68]. Therefore the concrete in such an enviromnent is readily corroded. However, concrete from calcium aluminate cement is stable with no special protection, at pH of water solution reduced to 4 [69]. As it was aforementioned, beside of pH value there are the other factors involved diffusion rate (permeability) and, as in all the chemical reactions, the solubility of products. 

Recent developments in low- and ultralow-cement castables/pumpables, the effects of the ultrahne particles are of great significance. The water requirements are low since the ultrafine silica fume particles (mostly used in these compositions) occupy part of the space of water. Although the silica fiune helps in reducing water requirements in the castable, it affects high-temperature properties due to the formation of anorthite and gehlinite phases at temperatures around 1250°C- 1400°C. The effects are somewhat minimized in ultralow cement castables due to the use of lesser quantity of calcium aluminate cement. 

Dry mix mortars often exhibit a quite complex mix composition, especially if they are rapid setting and/or rapid hardening. In the latter case, they generally contain binary or ternary binders based on calcium aluminate or calcium sulfoaluminate cements in blends with calcium sulfate without and with portland cement. Isothermal calorimetry is an efficient method to use for optimising mix designs of such mortars with respect to the hydration kinetics. As only small cement mortar or paste samples are used, the influence of the binder composition as well as of different combinations of accelerators, retarders, water reducers, plasticisers, etc. can quickly be tested. Two examples of how the amount of calcium sulfate addition is able to influence hydration kinetics are shown for blends of calcium aluminate cement with hemihydrate (Figure 2.22) and ternary binders based on port-land cement, calcium sulfoaluminate cement and anhydrite (Figure 2.23). 

Use of gypsum in order to reduce the rate of calcium aluminate ration with water, as a set controlling agent is as old as Portland cement productioa It is commonly accepted that the CjA hydration is hampered by the ettringite layer covering the aluminate crystals and forming an impermeable barrier [30], This old hypothesi s has been proved recently [31-33]. 

Many substances are known to act as accelerators for concrete. These include soluble inorganic chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates, thiosulfates, silicates, aliuninates, alkali hydroxides, and soluble organic compounds such as triethanolamine, calcium formate, calcium acetate, calcium propionate, and calcium butyrate. Some of them are used in combination with water reducers. Quick setting admixture s used in shotcrete applications and which promote setting in a few minutes may contain sodium silicate, sodium aluminate, aluminum chloride, sodium fluoride, strong alkalis, and calcium chloride. Others are solid admixtures such as calcium aluminate, seeds of finely divided Portland cement, silicate minerals, finely divided magnesium carbonate, and calcium carbonate. Of these, calcium chloride has been the most widely used because of its ready availability, low cost, predictable performance characteristics, and successful application over several decades.In some countries the use of calcium chloride is prohibited, in some others, such as Canada and the USA, the use of calcium chloride is permitted provided certain precautions are taken. Attempts have continued to find an effective alternative to calcium chloride because of some of the problems associated with its use. 

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